Update GameManager package/class to new name

[Algebro7]

GameManager in Megamek was refactored into megamek.server.totalwarfare.TWGameManager recently, which caused MekHQ to fail to compile. There were only two instances I found that needed to be updated, and all MekHQ unit tests were still passing.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.80%. Comparing base (20ea3a7) to head (3aae92e).
Report is 59 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4778      +/-   ##
============================================
+ Coverage     10.68%   10.80%   +0.12%     
- Complexity     5709     5714       +5     
============================================
  Files           950      950              
  Lines        131268   131306      +38     
  Branches      19136    19139       +3     
============================================
+ Hits          14021    14193     +172     
+ Misses       116022   115886     -136     
- Partials       1225     1227       +2     

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